Alumina-supported CO hydrogenation catalysts prepared from molecular osmium and ruthenium clusters

Abstract

Os catalysts on γ-Al₂O₃ supports have been prepared from Os complexes of varying nuclearity, namely H₂OsCl₆, Os₃(CO)₁₂, H₄Os₄(CO)₁₂ and Os₆(CO)₁₈. Characterization by infrared and X-ray photoelectron spectroscopy and transmission electron microscopy provides evidence of the stabilization of well-defined ensembles of Os atoms on the support surface, the ensemble size being determined by the nuclearity of the cluster precursor. The catalysts prepared from H₂OsCl₆ have a dispersion comparable with that obtained with Os₃(CO)₁₂, although the presence of smaller ensembles and single atoms cannot be excluded. The ensemble size may influence activity and selectivity for C₂ and higher hydrocarbons in CO hydrogenation at atmospheric pressure and temperatures between 530 and 610 K, but the results suggest that the heterogeneity of the Os species and the chlorine content of the support also influence the catalyst performance. Data obtained with a more highly dispersed Ru/Al₂O₃ catalyst prepared from Ru₃(CO)₁₂ provide the first quantitative comparison between Os and another Group VIII metal catalyst for CO hydrogenation. The Os was approximately one order of magnitude less active than the Ru catalyst, but it was more selective for formation of C₂ and C₃ hydrocarbons.